BALTIMORE — It is becoming perhaps the most intriguing question of this Yankee season: How exactly did Chad Green, something of a fringe prospect as a starting pitcher, turn into a strikeout monster as a reliever?

Austin Romine, who caught him on Monday as Green blew away Orioles hitters for seven outs that changed the game in a 7-4 win, shrugged and basically said he’s just glad he doesn’t have to hit against him.

“We’re all looking for an answer,’’ Romine said. “His fastball just kind of jumps at hitters. He just comes at you and lets it eat.”

Green is not the first converted starter to throw with more velocity as a reliever. It worked for Phil Hughes during the Yankees’ 2009 championship season, and, to some extent, a similar move saved Dellin Betances’ career after failing as a minor-league starter.

In any case, Green throws 96-97 mph out of the pen, a couple of ticks harder than he did as a starter, because he knows he’s only going to throw 30 to 40 pitches.

That’s high gas, obviously, but in an era when high-90s velocity is now common, and indeed everyone is trying to figure out why Aroldis Chapman isn’t getting a high number of swings and misses anymore, Green is proof that the number on the radar gun isn’t all that matters.

Late life is the term you hear all the time from scouts and players, and Green’s fastball obviously has it, which creates deception for hitters.

Chad Green has a 1.97 ERA and 90 strikeouts so far this season.

(Mike Stobe/Getty Images)

“It’s got some jump at the end, that’s the best way I can put it,’’ said Romine. “It keeps its velocity and it gets on you real quick. It gets on me sometimes when I’m catching him, so I know it gets on hitters.”

Even as a starter Green had a good fastball, but nothing like this. With four strikeouts in his 2 1/3 hitless innings on Monday, the 26-year old righthander now has 90 strikeouts in 59 1/3 innings.

And he gives the Yankees a weapon that few, if any teams, have to shut down offenses as early as needed and get the game to the late-inning relievers.

Without Green on Monday, in fact, who knows how differently this game might have turned out?

Jordan Montgomery had settled down some after a rough start, but baseballs were flying on both sides and the game had the feel of one of those classic Camden Yards slugfests.

Then, with two outs in the fifth inning, the Yankees leading 5-3, Montgomery walked Manny Machado and Girardi wasn’t waiting any longer to go to Green.

Chad Green’s emergence has allowed manager Joe Girardi to go to his bullpen early in games.

Some managers would have given Montgomery another hitter, trying to get him a win. But given the time of the year and the state of the wild-card race, Girardi was right to be proactive, with No. 3 hitter Jonathan Schoop coming up and the Orioles swinging hot bats as a team these days.

In any case, Green made him look awfully smart, setting down seven straight hitters, the last three swinging at his fastball.

“His fastball plays up out of the bullpen,” Girardi said. “And similar to Dellin, it has been simplified for him. He doesn’t need to be a three or four-pitch pitcher. He’s a two-pitch guy and it’s been successful.”

So Green got the ball to David Robertson, then Betances, and with the help of home runs by Didi Gregorius and Starlin Castro, the Yankees turned an early 3-0 deficit into a drama-free victory in a ballpark where they haven’t won a series since 2013.

Maybe they’ll change that over the next two games. Suddenly they have a confident vibe about them reminiscent of the way they were playing early in the season, perhaps because this is as healthy as their lineup has been in months.

Chad Green has increased his velocity since moving to the bullpen.

(Elsa/Getty Images)

Yes, the offense is beginning to click again. Even with Gary Sanchez out for this series, as he serves his suspension for the brawl in Detroit, the Yankees rocked Dylan Bundy, who was coming off a one-hit shutout against the Mariners.

Where all of this leads remains to be seen, but the depth of the bullpen is what could make them especially dangerous in the post-season. Even if Chapman doesn’t re-discover his dominance, Robertson and Betances are a strong finishing duo, and with the off days in October, Green could be even more of a factor _ perhaps an Andrew Miller-like wild card.

“With the weapons we’ve got, we can really shorten games,’’ was the way Romine put it.

Green’s emergence could have ramifications beyond this season. As I wrote Sunday, scouts are saying he could easily turn into a late-inning reliever, which would allow the Yankees to package someone like Betances, who is two years away from free agency, and a prospect for another quality starting pitcher.

But that’s for the offseason. For now Green, seemingly out of nowhere, has become a vital part of the formula for trying to win a championship.

How? Green smiles and says he doesn’t feel much difference in what he’s doing compared to previous years. So maybe Romine’s explanation will have to suffice: